Polyesters such as polyethylene terphthalate (PET), polyethylenenaphthalate (PEN), and similar polymers and copolymers have become staple commodities whose manufacture is well known and mature. Typical PET polymer preparation, for example, includes an uncatalyzed esterification of ethylene glycol with terephthalic acid, followed by increasing degrees of polycondensation. The polycondensation is catalyzed, and the last portion of this polycondensation is termed by some as “high polymerizing,” and by others as “finishing,” the term we will use herein. The finishing stage involves the highest temperature, usually 270° C. to 300° C., and the lowest pressure. The polymer obtained is extruded into water to form amorphous pellets, which are then subjected to crystallization. The polyester polymer product generally has a molecular weight which is too low for many applications, such as beverage bottles. The molecular weight, as reflected by the polymer's intrinsic viscosity, is generally in the range of 0.40 dL/g to about 0.67 dL/g. The polyester also contains measurable amounts of acetaldehyde generated during polymerization, as well as acetaldehyde precursors which may liberate acetaldehyde during later melt processing, such as the injection molding of bottle preforms.
The polyester pellets obtained from the finisher are thus subjected to solid state polymerization at a temperature below the melting point, and preferably in the range of 180° C. to 220° C., in a vacuum or a stream of inert gas. “Solid stating” has two principle advantages. First, it significantly increases the molecular weight, for example to an intrinsic viscosity in the range of 0.7 dL/g to 1.1 dL/g; and second it removes acetaldehyde from the polymer. However, these advantages are not obtained without significant cost: solid state polymerization is both energy and time intensive. Solid state polymerization also creates within pellets a large core to surface molecular weight gradient, which results in significant intrinsic viscosity loss upon melting the pellets. Eliminating solid state polymerization would be of great economic benefit.
Choice of polycondensation catalyst is important in polyester production, and many catalyst systems have been proposed. Titanium catalysts are the most efficient, and offer high polycondensation rates at low catalyst levels. However titanium catalyzed PET generally has both a high level of acetaldehyde as well as increased acetaldehyde generation during later melt processing. Relatively small amounts of acetaldehyde in beverage bottles can impart an off-flavor to the contents, and thus minimum acetaldehyde generation is quite important. Titanium catalysts also impart a yellow cast to the polyester.
Antimony catalysts have become the catalyst of choice, even though considerably less active than titanium. Phosphorus compounds are often added to improve moisture sensitive haze and thermal stability. If phosphorus stabilizers are not added, acetaldehyde generation rate upon melting the PET may be a concern, depending on the melt-phase conditions used. The AA generated upon melting is especially a concern when the polyester is manufactured exclusively in the melt-phase, that is, with no solid-stating. As disclosed in U.S. Pat. Nos. 5,750,635 and 5,886,133, the addition of phosphorous compounds can form precipitates which impact the clarity of the polyester. Some of the antimony catalyst is reduced to the metal under polymerization conditions. This results in dark PET, while brightness is valued in the industry.
Germanium catalysts have been proposed, but have not been entirely successful. For example, in U.S. Pat. No. 6,022,603, germanium dioxide is used in conjunction with compounds of cobalt, manganese, and magnesium, plus a phosphate stabilizer. The germanium catalyst is added after the phosphorus compound has been added to the melt, and after the intrinsic viscosity has reached 0.3 dL/g. However, the '603 patent emphasizes that in practical processes, intrinsic viscosity can only reach 0.50 to 0.67 during the finishing stage, and thus the polymer must be subjected to solid state polymerization to obtain useful products. A similar process, also involving solid state polymerization to a viscosity of 0.7 to 0.9 is disclosed in JP 2002 097353A. However, in the latter reference, the polyesters produced thereby had crystallization problems, and it was found to be necessary to add polytrimethylene terephthalate to induce crystallization, thus increasing the cost and complexity of the process.
In U.S. Pat. No. 6,590,044, antimony or germanium are disclosed in the alternative as polycondensation catalysts for PET/PEN copolymers. The polycondensation temperature is described as high and is followed by solid state polymerization. As prepared, prior to solid stating, intrinsic viscosity was 0.56 to 0.61 when germanium was employed as a catalyst.
An additional problem which is somewhat unique to germanium catalysis is the volatility of germanium compounds. Thus, at the high temperature and high vacuum of the prior art finishing processes, significant loss of germanium occurs. Since germanium is far more expensive than antimony, this loss is economically very disadvantageous. This expense, coupled with the need to solid state polymerize, has limited the use of germanium catalysts.